By far the greatest benefits of Latin and Greek accrue once one is more deeply involved in the grammar. If you are primarily after the vocabulary benefits, your proposal to go on to roots and beginning Greek is fine. If you want some of the other benefits, you are going to have to stick with it a little longer, probably, and then add Greek. If you are concerned about your 10yo’s ability, and want to just catch up a little, that is fine, but you might consider getting something for your older one to work on also.